Impulse testing apparatus



2 Sheets-Sheet 1 O 1947. R. K. GHORMLEY IMPULSE TESTING APPARATUS Filed Jan. 8, 1945 15 wi n vn mm- 5 on 1 3 3 8 -3 {x3 INVENTOR. ROGER K. GHORMLEY ATTORNEY Oct. 7, 1947. R. K. GHORMLEY IMPULSE TESTING APPARATUS Filed Jan. 8, 1945 2 Sheets-Sheet 2 INVENTOR. ROGER K. GHORMLEY ATTORNEY Patented Oct. 7, 1947 IMPULSE TESTING APPARATUS Roger K. Ghormley, Chicago, Ill., assignor to Automatic Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application January 8, 1945, Serial No. 571,781

15 Claims. 1 1

The present invention relates in general to apparatus for testing the operation of an impulsing contact and in particular to apparatus for remotely testing the operation of an impulse transmitter located at a subscriber substation of an automatic telephone system.

The operation of the switching equipment of an automatic telephone exchange depends upon both the speed and the impulse ratio of the impulses it receives from the transmitter at a subscriber substation. Heretofore. automatic testing apparatus has been provided to test the speed of operation of the impulse transmitter without the intervention of an operator but the assistance of a test operator at the exchange has been required to perform an impulse ratio test. In the copending application of Frank Kessler et al., Serial No. 571,782, filed Jan. 8. 1945, now Patent No. 2.416102, granted Feb. 18, 1947; an impulse testing apparatus operative under the control of the impulse transmitter at a subscriber substation to simultaneously perform an impulse ratio is to provide an improved automatic testing means operative under the control of the impulse transmitter at a subscriber substation to perform impulse ratio and speed tests on impulses received therefrom with a higher degree of accuracy than that heretofore possible.

A further object of the invention is to provide an impulse testing apparatus operative under the control of the impulse transmitter at a subscriber substation to perform an impulse ratio test on impulses received therefrom and in which the impulse ratio test is independent of the speed of the impulses.

Still another object of the invention is to increase the accuracy of apparatus utilizing the charge or discharge of a condenser to measure time intervals by arranging to charge or discharge a plurality of condensers in multiple during the time interval being measured and arranging to utilize the total voltage across the plurality of condensers connected in series to indicate the duration of the measured time interval.

Other objects of the invention will appear upon a further perusal of the specification taken in connection with the accompanying drawings comprising Figs. 1 and 2, which taken together form a complete schematic diagram of the testing apparatus.

It is contemplated that the testing apparatus may be made an integral part of a conventional telephone exchange or that it may be made as impulses.

2 a separate portable unit which may be connected to the exchange switching equipment as required. To this end the apparatus is made as complete in itself as possible. The only connections which need be made to the regular exchange equipment are two connections to the exchange battery and three connections to the bank contacts of a selector through which the testing unit may be reached by the exchange switching equipment. The three connections to the selector incude two line leads over which the subscriber loop is extended and the private, or holding lead, over which the switch train is held under the control of the subscriber loop circuit. The line leads terminate in an impulsing relay which is preferably of the same type and has the same impulsing characteristics as the corresponding line relays in the switching equipment. Contacts of this imp-ulsing relay are arranged to control a group of relays and a rotary switch to perform the desired tests in response to operation of the impulsing relay by the transmitter at a subscriber substation. The first series of operations of the impulsing relay causes the apparatus to select the desired limits between which the test is to be made. A succeeding series of a predetermined number of operations of the impulsing relay causes simultaneous tests to be made which det rmine whether the speed and impulse ratio are within the selected limits. The apparatus then returns a coded tone signal to the subscriber substation indicative of the results of the test and also indicative of whether the required predetermined number of impulses were received. The actual measurement of the speed and impulse ratio is performed by a series of condenser-resistor networks and associated gaseous discharge tubes. The speed measurement is accomplished by causing two groups of initially charged condensers to be partially discharged during the time that the predetermined number of operations of the impulsing relay takes place. The charge which remains on these condensers is then an indication of the total duration of the series of The remaining charge is measured by two gaseous discharge tubes and relays controlled by these tubes are operated in accordance with wh ther the remaining charge is within the previously selected limits. To increase the accuracy of the time measurement each group of condensers consists of four condensers which are discharged in multiple and then connected in series aiding to the gaseous discharge tube control electrodes. The impulse ratio measurement is accomplished by a further group of initially charged condensers. Each of these further groups is divided into two sub-groups. One subroup of each further group is partially discharged during the break periods of the received impulses while the other sub-groups of each further group are partially discharged during the time that the predetermined number of operations of the impulsing relay takes place. By the proper choice of circuit constants the charges on the two sub-groups of condensers of each further group can be made equal for a desired impulse ratio independently of the impulse speed. The difference between the charges on the subgroup condensers is measured by a pair of gaseous discharge tubes and relays controlled by these tubes are operated in accordance with whether the difference is positive or negative. The relays controlled by the gaseous discharge tubes prepare marking circuits to the banks of the rotary switch from a tone generator. The rotary switch is then operated to successively connect the subscriber line to the several marking circuits and thereby causes a coded tone signal to be returned to the subscriber substation indicative of the results of the speed and impulse ratio test.

Referring now to the drawings, a subscriber substation I is shown connected via line 2 to an automatic telephone exchange 3. By dialing an appropriate number over line 2 from the subscriber substation, line 2 may be extended by means of the exchange switching equipment to line 4. The usual impulse transmitter IT forming a part of the subscriber telephone may be employed for this purpose or, if it is so far out of adjustment as to preclude proper operation of the exchange switching equipment, a separate impulse transmitter such as is usually carried by telephone maintenance men may be used. When the subscriber loop is extended to line 4 a circuit is completed to relay A which operates and closes a circuit to relay B. Relay B operates, at contacts B5 grounds lead 5 to hold the switching equipment in the exchange through which the subscriber loop is extended, at contacts B3 connects ground or plus battery to the anode of tube 6 through the lower winding of relay L, and at contacts B6 closes a circuit to the series connected heaters of tubes 6, I, 8, and 9. A ballast tube ET is connected in series with the tube heaters to prevent changes in exchange battery voltage from affecting the heater current. During the warm up period this ballast tube is shunted by a resistor through contacts L5. The operation of relay B also completed a circuit from contacts B3, through contacts L2, and over lead 38 to the upper winding of relay P. Relay P is arranged to complete a circuit to its lower winding when it operates. Its two windings are connected differentially which causes it to vibrate its armature at a rapid rate and thus produce an audio frequency voltage across its lower winding. This alternating tone voltage is conveyed through condenser l5, over lead 37, and through contacts Ll to line 4 which is connected via the exchange switch train to the subscriber line 2. When the cathodes of the tubes have been heated sufficiently, tube 6 strikes and energizes the lower winding of relay L. Relay L operates, disconnects the line 4 from the tone source at contacts LI to inform the test man that the equipment is ready for use, opens the circuit to the tone generating relay P at contacts L2, closes a locking circuit to its upper winding at contacts L3, transfers the anode of tube 6 from the lower winding of relay L to lead 39 at contacts L4, and removes the resistance shunt from ballast tube BT at contacts L5. Tube 6 deionizes as there is no potential on lead 39 at this time.

The testing unit is now prepared for use. The test man at the subscriber station is iniormed of this fact by the secession of the tone and he therefore proceeds to dial an appropriate digit to select the limits between which he desires to test the impulsing device at the subscriber station. In the present instance three sets of limits have been provided, any one of which may be selected by dialing 8, 9, or 0. Assuming that the test man dials a 9, relay A will be momentarily released nine times in succession. Each time that relay A restores it completes a circuit to relay C which is thus momentarily operated nine times in succession. Each time that relay C operates it closes a circuit to the motor magnet M of the rotary switch from ground at contacts B3, through contacts C I over lead 2 I, and through contacts S2 to MM. Motor magnet MM advances the rotary switch wipers one step each time that it restores. A circuit is also completed to relay D by contacts C2 while relay C is operated. Relay D is rendered slow to release by a copper sleeve on its core and thus remains operated during the series of impulses. Relay B is similarly made slow to release so that it remains operated even though its circuit is interrupted by relay A during dialing. At the conclusion of the series of impulses the wipers WI to W4 of the rotary switch will be resting on the ninth set of bank contacts. Relay D restores shortly after the series of impulses is completed and closes a circuit to relay J from ground at contacts B3, over lead. 29, through contacts M5, over lead 21, through contacts DI and GI, over lead 30, through contacts M3 and the ninth bank contact of wiper W2, and over lead 23 to relay J. Relay J operates, closes its locking circuit at contacts J I to ground on lead 21, and closes a circuit in multiple With itself to relay G at contact J2. Relay G operates and at contacts GI opens the operating circuit to relay J and closes a homing circuit to the rotary switch motor magnet from ground on lead 2i, through contacts DI and GI, over lead 3!, and through cam springs CS and interrupter springs IS to MM. The motor magnet operates self-interrupting to drive the wipers to the eleventh step where the cam springs CS open to stop further rotation of the wipers. The operation of relay G also completed a charging circuit to all of the condensers in Fig. 2 at contacts G4. The condensers in the first and second horizontal rows are each charged to the exchange battery voltage. In the third and fourth horizontal rows the first four condensers from the left of each row are also each charged to the exchang battery voltage while the remaining two condensers at the right of each row are each charged to a predetermined fraction of the exchange battery voltage. The fractional voltage to which the latter four condensers are charged is determined by the relative magnitudes of resistors III and I l.

The test man now dials an 0 causing relay A to release and operate relay C ten times in succession. Relay D again operates the first time that relay C operates and remains operated throughout the series of impulses. The operation of relay C also completes a circuit to relay 0 through contacts C2, G3, and ZI. The operation of relay D opens the homing circuit to the rotary switch at contacts DI and closes a circuit to relay E at contacts D2 through contacts G2 to ground at contacts B3. Relay E operates; at contacts El closes a circuit to relay N over lead 28, at contacts E2 extends ground from contacts B3 to wiper W4 over lead 25, at contacts E4 closes a circuit to relays O and Z in series through contacts Q1, and at its lower six sets of contacts opens the charging circuits to all of the condensers in Fig.2 excepting the first two condensers from the right in the third and fourth rows. Relay N operates and looks over lead 32 to ground at contacts B2,

Relay Z is short circuited during the first operation of relay C but when relay C restores the first time this short circuit is removed at contacts 02. Relay Z operates in series with relay 0, opens its shunting circuit at contacts Zl, at contacts Z2 and Z3 closes discharge paths through the pair of resistors previously selected by the operation of relay J to the two condensers on the right of each of the first and second rows, at contacts Z4 and Z5 prepares discharge paths to the two condensers on the left of each of first and second rows, and at contacts Z6 and Z1 closes discharge paths through the second pair of resistors previously selected by the operation of relay J to the four condensers on the left of each of the third and fourth rows. During the succeeding nine operations of relay the discharge paths prepared by contacts Z4 and Z are intermittently completed by contacts C3 and C4 through resistors l2 and 13, respectively, The motor magnet MM is operated intermittently by contacts Cl over the previously traced circuit causing the rotary switch wipers to advance one step at the end of each impulse. When relay 0 restores for the tenth time wiper W4 is advanced to its tenth bank contact thus completing a circuit to relays FA, F3, F0 and FD in multiple over lead 3-5. Relays FA through FD operate, transfer all of the condensers to the control electrodes of their associated tubes in series connected groups, and close a circuit in multiple with themselves over lead 36 to relay Y. Relay Y operates, closes its locking circuit at contacts Y2 through con-' tacts NI and over lead 29 to ground at contacts B3, and connects plus battery from contacts E3 over lead 34 and through contacts Y3 and the lower windings of relays T, U, V, and X to the anodes of tubes 6, 1, 8, and 9, respectively, over leads 39, 40, 4|, and 42.

First considerin only the first horizontal row of condensers associated with tube 6, during the testing interval the two condensers on the left are discharged during the break periods of nine of the ten impulses received and during the same period the two condensers on the right are discharged continuously. Let V0 be the initial voltage to which all of these condensers were initially charged, R1 be the resistance through which the left two condensers ar intermittently discharged, C1 be the capacity of each of these condensers, r be the ratio between the break periods to the total period of each of the received impulses, R2 be the resistance through which the right two condensers are continuously discharged, C2 be the capacity of each of these condensers, and t be the total time required for the nin impulses during which the condensers are discharged. Then the final voltage V1 across the left condensers will be where 5 271828 and the final voltage v2 across the right two condensers will be regardless of the value of the initial voltage V0 to which the condensers were initially charged or of th total time it during which the discharge takes place. When relay FA operates it connects the four condensers in series to the control electrode of tube 6 so that the voltage between the control electrode and cathode is equal to 2(V2V1) plus a fixed bias potential. The fixed bias potential is derived from a voltage divider connected from ground at contacts G4 to negative battery and having a tap connected to the oathode and shield grid of tub 6, The value of this bias potential is made equal to the striking potential of the tube so that when V1 is greater than V2 the tube will fire and when V1 is less than V2 the tube will not fire. Thus for given values of RiCi and RzCz tube 6 will fire if the impulse ratio r "is below a particular value and tube 6 will not fire if the impulse ratio is above this particular value. It should be noted that since the voltages V1 and V2 are equal for a particular impulse ratio regardless of the value of the time t during which the discharge takes place, the ratio test is independent of the impulse speed. Furthermore, the voltages V1 and V2 are equal for a particula impulse ratio r regardless of the value of the initial voltages V0 to which the condensers were initially charged, thus by operating tube 6 over the range where its grid striking potential is a linear function of its anode voltage the effects of variation in exchange voltage are eliminated since the bias voltage and plate voltage are derived from the same source and vary in the same proportions. It should be apparent that if n condensers were discharged in multiple in each of the two paths and then connected in series that the net grid voltage would be n(V2V1) thus permitting still greater sensitivity without any increase in the voltage to which the condensers are initially charged.

The second horizontal row of condensers and the associated tube I operate in precisely the same manner as described in the preceding paragraph except that a different ratio of is used so that whether tube I fires or not is determined by whether the impulse ratio r is below or above a different particular value. Thus if the impulse ratio is below both particular values both tubes will die, if the impulse ratio is between the two values only one tube will fire, and if the inn pulse ratio is above both values neither tube will fire. In the present instance tube 6 is the one that normally fires.

Referring now to the third horizontal row of condensers and the associated tube 8, th four condensers on the left are continuously discharged in multiple during the time that nine of the ten impulses are received but the two condensers on the right are not discharged. When relay FC operates these six condensers are connected in series between the control electrode and cathode of tube 8. The two right condensers are connected in opposition to the four left condensers. The time constant RC of the discharge path of the four left condensers is preferably made equal to the duration of nine impulses at a desired particular speed. The potential to which each of the right two condensers are charged is then made equal to twice the final voltage of each of the four left condensers in a time equal to the time constant RC. Thus the net voltage across the six condensers in series will be zero if the duration of the nine received impulses is equal to RC. If a bias potential was used in the grid circuit of tube 8 as for tubes 6 and I the tube 8 would just strike when the voltage across the six condensers was zero. In this case it is not necessary to employ a bias potential since the voltage to which the two right condensers are charged can be increased slightly to provide a net Voltage, when connected in series with the four right condensers, equal to the striking voltage of tube 3. This difference voltage will be a linear function of the exchange voltage and will therefore vary in direct proportion to the anode voltage of the tube when the exchange battery voltage varies thus eliminating any variations due to changes in exchange battery voltage. This method of operation could not be used in the impulse ratio test because if the bias voltag was eliminated and instead voltages V1 and V2 were allowed to be unequal at the particular impulse ratio at which the tubes 6 or l were to fire, then the difference between V1 and V2 would be a function of both the exchange voltage and duration of the discharge time. Variations due to changes in exchange voltage would still be eliminated but the ratio test would not be independent of the impulse speed. The use of the two right condensers in the speed test permits comparison of the final voltage of the four left condensers with a voltage equal to a predetermined fraction or multiple of the exchange battery voltage and yet permits the exchange battery to be used as the anode voltage supply for the tube 8. If the difference between four times the final voltage of each of the four left condensers and twice the voltage of each of the two right condensers is above the grid striking potential of tube 8 it will fire, while if this difference is below the grid striking potential it will not fire. Thus if the impulse speed is faster than a particular value tube 8 will fire and if the speed is slower than the particular value tube 8 will not fire.

The fourth horizontal row of condensers and associated tube 9 operate in precisely the same manner as described in the preceding paragraph except that a different time constant is used so that tube 9 will fire if the impulse speed is faster than a different particular value and will not fire if the impulse speed is slower than the different particular value. Thus if the speed of the received impulses is faster than either of two predetermined values both tubes will fire, if the speed is between the two values only one tube wil1 fire, and if the speed is slower than either value neither tube will fire. In the present instance it will be assumed that tube 8 is the one that normally fires Assuming that the impulse ratio and speed of the received impulses were within the previously selected limits, tubes 6 and 8 fire and their anode currents flowing over leads 39 and 4| nergize the lower windings of relays T and V. Relays T and V operate, prepare marking circuits to the bank contacts of wiper WI at contacts TI and VI, close locking circuits to their lower windings at con tacts T2 and V2, and open the anode circuits to tubes 6 and 8 at contacts T3 and V3. Tubes 6 and 8 deionize. This completes the registration of the results of the test, the next step being the return of a coded tone signal to the test man indicative of the results of the test.

Relay D restores shortly after the series of ten impulses is completed and at contacts DI closes the homing circuit for the rotary switch which was previously traced, and opens the circuit to relay E at contacts D2. Relay E restores and opens the series circuit to relays O and Z which restore. The motor magnet MM advances the rotary switch wipers to the-eleventh step where a circuit is completed from ground at contacts B3, through contacts E2, over lead 26, through the eleventh bank contact of wiper 3 and contacts N3 to relay M. Relay M operates; opens the homing circuit to the rotary switch, opens the locking circuits to relays J and G, and closes a circuit to relay R at contacts M5; prepares its locking circuit at contacts M4; closes a circuit to the tone generating relay P at contacts M2; and closes a multiple circuit to contacts NI at contacts MI. Relays J and G restore. Relay P begins to generate a tone voltage. Relay R operates and closes a circuit to relay S at contacts R2. Relay S operates, short-circuits relay R at contacts SI, and closes a circuit to motor magnet MM at contacts S2 in multiple with itself. Motor magnet MM operates. Relay R restores, opens the circuits to relay S and MM and short-circuits relay S at contacts R2. Motor magnet MM restores and advances the rotary switch wipers to the first set of bank contacts. Wiper W3 opens the operating circuit and closes the locking circuit to relay M through contacts M4. Relays R and S continue to pulse. These two relays are made slow to operate and to release by means of copper slugs on the armature ends of their cores to regulate the speed at which they operate. During a portion of the time that the wiper W3 rests on bank contact I, a circuit is completed from the lower winding of relay P, through condenser I5, contacts RI, wiper WI, over lead 33 to line 3, and thence to the subscriber line 2 over the exchange switch train; thus returning a splash of tone to the subscriber telephone. Each time that relay S operates, a circuit is completed to the motor magnet MM causing it to operate, and each time that relay R restores it opens the circuit to the motor magnet MM causing it to restore and advance the wipers one step. While the wipers are resting on the second step another splash of tone is returned to the subscriber telephone, during the interval when relay R is operated, over a circuit similar to that previously traced except that it now includes contacts TI. Relay N is short-circuited through wiper W2 and contacts M3 causing it to restore. A long splash of tone is returned when the wiper WI rests on bank contact 5 and 6 because these contacts are multipled and the tone circuit is not interrupted by relay R. This circuit is taken through contacts YI so that the presence of the long splash of tone serves to verify to the test man that the digit 0 was correctly received by the testing apparatus. Two more short splashes of tone are returned when the wipers rest on the ninth and tenth bank contacts. The first circuit includes contacts VI and RI and the second in cludes only RI. Thus the tone signal received by the test man consists of two short splashes of tone followed by a long tone signal and then two more short splashes of tone. When the rotary switch is advanced to the eleventh step the locking circuit to relay M is opened by wiper W3. Relay M restores, at contacts MI opens the looking circuits to relay FA, FB, FC, FD, Y, T, and V causing them to restore, opens the circuit to the tone generating relay P at contacts M2, and opens the circuit to the interrupter relays R and S at contacts M5. All relays are now restored to normal except relays A, B, and L which remain operated until the subscriber loop circuit is opened by the restoration of the receiver on the hookswitch of the telephone at the subscriber station. Since the equipment is now in exactly the same condition as it was prior to the test, it should be apparent that the test may be repeated by reoperati-ng the impulse transmitter at the subscriber substation. If desired, the digits 8 and 0, or and 0, may be dialed the second time in place of 9 and 0 in order to change the limits. between which the tests are 'made. If no further tests are required the test 'man restores the receiver on the hookswitch causing the subscriber loop circuit to be opened. When the subscriber loop circuit. is opened relay A restores and opens the circuit to-relay B which restores and removes ground from the holding lead at contacts BS-and opensthe locking. circuit to relay L at contacts B3. Relay L restores.

In the event that the impulse ratio r of the received impulses was too low, or conversely the ratio (1-1") of the closed to the total periods of the received impulses was toohigh, both tubes 6 and 1 would have fired causing the operation of both relaysT and U. Bank contacts I, 2, and 3, of wiper WI would. then be connected to the tone source through. contacts RI causing three short splashes of tone. to-be returned to the test man before the long tone signal. If the impulse ratio r was too-high, or conversely the ratio (l-r) of the closed to. the total periods, of the received impulses. was too low, neither tube 6 nor 1 would have fired. and consequently neither relay T nor U would have been operated. Thus only one short splash of. tone would be sent before the long tone signal. In a similar manner, it the dial speed was too fast both tubes 8 and 9 Would fire causing both relays V and X tooperate, or if the dial speed was tooslow neither tube 8 nor 9 would fire causing both relays V and X to remain normal. Thus three splashes of tone following the long tonesig-nal, indicating a fast dial; or One splash of tone following the long tone signal, indicating a slow dial; would bereturned to the testman.

It is oftentimesdesirable to have different adjustment margins. for different dials depending upon the type of line with which they are used. Thus a wide marginmaybe permissible for a local subscriber dial, an. intermediate margin may be required. for dials. which are used. with short trunks or heavily. loaded party lines, while a narrow margin may be necessary for dials which are used with inter-ofilcetrunks. Provision for testing with any one of three margins is provided by arranging thetesting apparatusso that any one of the three relays. H, J, or K may be operated when the test man dials the first digit after seizing the testing equipment. In the previous discussion it was assumed that a 9 was dialed which caused the operation of relay J. It should be apparent from the precedingdescription that if the first. digit had been 8 or"0 instead-of 9, either relay K or Hwouldhave beenoperated in place of relay J. The operations are identical to those previously described except that slightly difierent valued resistors-are connected in the four condenser discharge paths. All of theseresistorsare adjustable so that the impulse ratio and speedlimits for any'oi thethree sets of margins can be varied as desired. These resistors may be initially adjusted by; operating relay'A with impulses of known characteristicaas from a varying machine and adjusting the various resistors until the desired resultsare obtained.

It should be understood. that numerous modi-- fications init'he details. or; the; circuit arrange- 10 ments may be resorted to without departing from the true spirit and scope of the invention asdefined in the subjoined claims.

What is claimed is:

1-. In a testing apparatus, asource of impulses, apair of condensers, means controlled by said source for altering the charge on one of. said condensers during a portion of each of a plurality of impulses and for altering the charge on the other of said condensers continuously and pro-- gressively during the whole of the plurality of impulses, and means controlled by the difference between the finalcharges on said condensers for indicating the ratio between said portion and a whole impulse.

2. Ina testing apparatus, a source of impulses, apair of initially charged condensers, means controlled by said source for discharging one of said condensers during a portion of each of a plurality of impulses and for discharging the other of said condensers continuously and progressively during the whole of the plurality of impulses, and means controlled by the difierence between the remaining charges on said condensers for indicating the ratio between said portion and a whole impulse.

3-. An arrangement for testing the impulse ratio of an interrupting device comprising a pair of condensers, means for altering the charge on one of said condensers to an extent determined by both the impulse ratioand speed of the device and for altering thecharge on the other of said condensers to an extent determined solely by the speed of the device independent of the ratio, and means controlled by the difierence between the charges on said condensers for indicating the impulse ratio or the device, whereby the impulse ratio indication is renderedv independent of the speed.

4. Anarrangement for testing the impulse ratio of an interrupting device comprising a pair of initially charged condensers, means for discharging one of said condensers to an extent determined by both the impulse ratiov and speed of.

the device and for discharging the other of saidcondensers to anextent determined-solely by the speed of the device independently of the ratio, and means controlled bythe difierence between the charges on said condensers for indicating the I impulse ratio of the device.

.5. Ina system for testing the impulse ratio of an interrupting device, a pair of circuits each including. a condenser, means controlled by the interrupting device for altering the charge on one of the condensers during a portion-of each of a plurality ofimpulses; and for altering the charge on theother ofthecondensers continu-- ously and progressively during the whole of the plurality of impulses; the time constants of said circuits having valuessuch that the final charges charges on. the. condensers -will-beequal for a 1 1 particular impulse ratio, and means for indicating the difference between the remaining charges on the condensers.

7. In a system for testing the impulse ratio of an interrupting device, four circuits each including a condenser, means controlled by the interrupting device for altering the charges on first and second ones of the condensers during a portion of each of a plurality of impulses and for altering the charges on third and fourth ones of the condensers continuously and progressively during the whole of the plurality of impulses, the time constants of said circuits having values such that the final charges on the first and third condensers will be equal for a particular impulse ratio and such that the final charges on the second and fourth condensers will be equal for a different particular impulse ratio, and means controlled by the difference in the charges on the first and third condensers and the difference in the charges on the second and fourth condensers for indicating whether the impulse ratio of the interrupting device lies between said two particular impulse ratios.

8. In a system as claimed in claim 7, means for at times changing the time constant of at least one of said circuits so as to change the value of one of said two particular impulse ratios.

9. In a system for testing the impulse ratio of an interrupting device, four circuits each including an initially charged condenser, means controlled by the interrupting device for discharging first and second ones of the condensers during a portion of each of a plurality of impulses and for discharging third and fourth ones of the condensers continuously and progressively during the whole of the plurality of impulses, the time constants of said circuits having values such that the remaining charges on the first and third condensers will be equal for a particular impulse ratio and such that the remaining charges on the second and fourth condensers will be equal for a different particular impulse ratio, and means controlled by the difference in the remaining charges on the first and third condensers and the difference in the remaining charges on the second and fourth condensers for indicating whether the impulse ratio of the interrupting device lies between said two particular impulse ratios.

10. In an electrical measuring apparatus, a time interval divided into two characteristic por tions, 2. pair of condensers, means for altering the charge on one of said condensers during one characteristic portion of said time interval and for altering the charge on the other of said condensers continuously and progressively during the whole time interval, and means controlled by the difference between the final charges on said condensers for indicating the ratio between said one characteristic portion and the whole time interval, whereby said indication is rendered independent of the duration of the time interval.

11. In a device, a voltage responsive means, two groups of condensers, means for connecting said condensers in parallel, means for altering the charge on each condenser in said groups of condensers connected in multiple during a given time interval, and means for connecting said groups of condensers in series to said voltage responsive means at the termination of said charge alteration, said voltage responsive means controlled by the difference of potential between the two groups of condensers in accordance with the final charges on said two groups of condensers.

' 12. In a device, a voltage responsive means, two groups of initially charged condensers, means for connecting said condensers in parallel, means for discharging each condenser in said groups of condensers connected iii-multiple during a given time interval, means for connecting said groups of condensers in series to a voltage responsive means at the termination of said discharges, said voltage responsive means controlled by the difference of potential between the two groups of condensers in accordance with the final charges on said two groups of condensers.

13. In a device, two groups of condensers, means for connecting said condensers in parallel, a plurality of means for discharging one of said groups of condensers at different rates, means for selecting one of said plurality of means and connecting said one group of condensers to said means, means for discharging the other of said groups of condensers at a different rate, means for connecting said groups of condensers in series to a voltage responsive means at the termination of said discharges, said voltage responsive means controlled by the difference of potential between the two groups of condensers in accordance with the final charges on said two groups of condensers.

14. In combination, a thermionic tube having at least a cathode, a control grid, and an anode, a source of potential connected between the cathode and the anode, a source of control voltage, a condenser, a first means for charging said condenser from said first source, a second means for discharging said condenser independent of said second source, and a third means operative to connect said condenser between the control grid and the cathode of said tube in series with said second source at the termination of said discharge, thereby producing a bias on said tube, which is the difference between the final potential on the condenser and the potential of said second source.

15. In a combination as claimed in claim 14, a second condenser, said first means arranged to charge said condensers in multiple from said first source, said second means arranged to discharge said condensers in multiple, said third means operative to connect said condensers between the control grid and the cathode of said tube in series with said second source at the termination of said discharge, thereby producing a biasing effect on said tube,

ROGER K. GHORMLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,168,198 Frink Aug. 1, 1939 2,274,992 Nelsen Mar. 3, 1942 1,907,279 Blomberg May 2, 1933 1,819,999 Kearsley Aug. 18, 1931 1,917,418 Almquist et al. July 11, 1933 1,992,908 Crockcroft et al. Feb. 26, 1935 2,078,618 Springer Apr. 27, 1937 1,577,046 Miller Mar. 16, 1926 1,759,873 Brown et a1 May 27, 1930 1,964,526 Melsheimer June.26, 1934 2,207,513 Hadfield July 9, 1940 2,245,683 Kessler June 17, 1941 FOREIGN PATENTS Number Country Date 522,808 Great Britain June 27, 1940 

